SUMMARY Alzheimer's disease (AD) is a complex cascade of neurodegenerative processes that expressed as gradual accumulation of A? peptides and hyperphosphorylated tau (ptau). The single most important factor is aging. However, mechanisms that mediate aging-associated risks are not well understood. Loss of synaptic structure and function marks early stages of AD that may precede clinical symptoms by decades. This implies that mechanisms of aging-related risks might be at work already by middle age. Explicit testing of such mechanisms in AD mouse models has been problematic, since in conventional models the effects of aging cannot be separated from the effects of advanced pathology. To address this challenge, we have employed inducible models that can conditionally delay expression of tau and/or APP until early adulthood, middle, or old age. We find that middle-age onset APP mice, which model initiation of amyloidosis in humans in the 4th decade of life develop more severe cognitive deficits and at earlier stages of A? accumulation than mice with an early-adulthood onset. Here, we will further employ these conditional models to search for molecular mechanisms that underlie age vulnerability. As part of this analysis we will explore the contribution of a candidate gene pathway, NPTX2, which was first implicated in our studies of human AD. Preliminary studies in mouse models demonstrate A dependent down-regulation of NPTX2 in middle and old age but not young adult mice. Accordingly, we hypothesize that NPTX2 represents an aging-sensitive pathway that is relevant to the pathophysiology of human AD. In Aim 1, we will use inducible transgenic mice with different ages of onset of APP and/or tau expression (young-adult, middle- or old-age) to test whether tau alone or in combination with A causes a reduction of NPTX2 expression. In Aim 2, we will screen for aging and A/tau associated changes in synaptic protein expression by quantitative mass spectrometry. Verified changes in NPTX2 and other synaptic proteins will be used to confirm sensitivity of MS analyses. In Aim 3, we will test whether age- associated downregulation of NPTX2 contributes to cognitive phenotypes. Aim 4 will assess the role of aging, A/tau and NPTX2 in depression phenotypes. Aim 5 will address possible sex-related differences in the aging- associated cognitive and depressive phenotypes. The studies planned in this proposal are designed to delineate the function and regulation of NPTX2-dependent pathways in a setting of progressive aging and AD pathologies and will help to reveal the molecular basis and therapeutic directions for cognitive and behavioral symptoms of AD.